This invention relates to electronic article surveillance systems and, in particular, to electronic article surveillance systems in which magnetic radiation or energy is used to carry out the article surveillance.
Electronic article surveillance systems are known in the art wherein surveillance is carried out by transmitting a magnetic field into an interrogation zone. In these systems, determining the presence of the articles under surveillance is accomplished by sensing perturbations to the transmitted magnetic field. These perturbations are generated by tags attached to or incorporated into the articles. These tags carry or are formed from magnetic markers or materials which create the perturbations.
In designing article surveillance systems of this type, attention is focused on achieving certain characteristics or criteria which are of importance to the user. One characteristic is referred to as the system "width." This characteristic defines the maximum width of the interrogation zone which can be used, while still detecting articles carrying valid tags with reliability. It is desirable that the system width be maximized so as to give the widest possible interrogation zone. This makes the surveillance system adaptable to a greater number of user locations.
A second characteristic is referred to as the system "pick." This is a measure of the percentage of time that the system identifies articles bearing valid tags. It is important that this characteristic also be maximized in order for the system to operate credibly as a surveillance system.
A third characteristic is the system "false alarm" rate. This characteristic is a measure of the percentage of time the surveillance system alarms as a result of objects other than articles carrying valid tags. Frequently, false alarms are brought about by metal objects such as shopping carts or watches passing through the interrogation zone. It is essential that the false alarm rate of the surveillance system be minimized to likewise promote system credibility as well as to avoid embarrassment to the user of having the system alarm for objects other than those under surveillance.
Other characteristics of the surveillance system of interest to the user involve the ability of the system to operate properly with tags which are deactivatable and with fixed metal objects in the floors, walls and other equipment in or bordering the interrogation zone. In the case of deactivatable tags, it is desired that the system alarm only for articles carrying tags which are in their active state. In the case of fixed metal objects, it is desired that the system be substantially immune to the otherwise masking effects of these objects.
The magnetic article surveillance systems designed to date have had difficulty in achieving all these characteristics. These systems generally fall into two categories. In one category of system, the antenna or coil used to transmit the magnetic field into the interrogation zone (the "transmitter coil") and the antenna or coil which receives magnetic energy from the zone (the "receiver coil") are disposed in spaced housings or pedestals which border the interrogation zone. This category of system is referred to as a "transmit/receive" system. Because of the spacing between the transmitter and receiver coils of the transmit/receive system (usually this spacing defines the system width), there is a relatively low coupling of magnetic energy therebetween.
In using the transmit/receive system, it has been found that acceptable pick rates are achievable and that low false alarm rates are also achievable, but only for objects with low masses such as, for example, deactivated tags and watches. This low false alarm rate for low mass objects is due to the aforementioned low coupling between the transmitter and receiver coils. Such low coupling causes the receiver coil sensitivity to be relatively low in areas where the transmitted energy is high. Hence, received energy due to low mass objects is usually insufficient to meet the criteria established for determining the presence of articles carrying valid tags. However, even with this low sensitivity of the receiver coil, high mass objects are found to generate sufficient received energy to meet these criteria, thereby causing false alarms.
A second category of system used for magnetic article surveillance is referred to as a "transceiver" system. In this type of system, transmitter and receiver coils are housed in common pedestals at each of a number of locations bordering the interrogation zone. Because of this common housing of the transmitter and receiver coils in close proximity to each other, there is a high coupling of magnetic energy therebetween.
In using the transceiver system, it has been found that the system provides a good pick rate for articles carrying single tags. It has also been found that the system provides a relatively low false alarm rate, but only for high mass objects such as, for example, shopping carts. This low false alarm rate for high mass objects occurs because of the high coupling between the transmitter and receiver coils. This coupling causes the receiver coil to be highly sensitive in the same areas where the transmitter energy is high. As a result, in the presence of high mass metal objects, the received energy becomes sufficient to meet the criteria established for identifying metal objects, before the criteria for identifying articles carrying valid tags are met. The high mass objects can thus be detected before the system alarms.
The transceiver system, however, provides a relatively higher false alarm rate for low mass objects, e.g., deactivated tags, watches, etc. Low mass objects are found to result in sufficient received energy to meet the criteria for identifying articles carrying valid tags, before the criteria for metal objects are met. Hence, the system is likely to false alarm for these low mass objects.
The transmit/receive and the transceiver systems can be modified in certain conventional ways to attempt to enhance their operation. Thus, to decrease false alarms and sensitivity to fixed metal objects, the transmitted energy can be decreased by decreasing the current to the transmitter coil. Decreasing the transmitted energy, however, decreases the system width and/or pick rate. Also, the criteria for differentiating between received signals indicative of articles carrying valid tags and those indicative of other metal objects can be varied or changed to provide some limited improvement in the pick and false alarm rates. Finally, placing more than one tag in an article can be used to increase the pick rate.
The above techniques for enhancing system operation have thus provided only limited improvement in system performance. Hence, there is still a need for a system which can provide a relatively wide system width, a relatively high pick rate and a relatively low false alarm rate for objects of low and high masses.
It is therefore an object of the present invention to provide an electronic article surveillance system having enhanced performance.
It is a further object of the present invention to provide an enhanced electronic article surveillance system which utilizes magnetic energy and tags employing magnetic markers.
It is a further object of the present invention to provide a magnetic electronic article surveillance system in which the system false alarm rate can be relatively low for objects of different masses.
It is yet a further object of the present invention to provide a magnetic electronic article surveillance system in which the system false alarm rate can be relatively low for objects of high and low masses and in which the system width and pick rate can be relatively high.